1. Field of the Invention
The present invention relates to wireless communication systems, wireless communication apparatuses, wireless communication methods, and computer programs for carrying out communications mutually among a plurality of wireless stations in a wireless local area network (LAN), a wireless personal area network (PAN), or the like. For example, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program for carrying out communications according to the IEEE 802.11n while maintaining compatibility with the IEEE 802.11a/g.
More specifically, the present invention relates to wireless communication systems, wireless communication apparatuses, wireless communication methods, and computer programs for executing a packet exchanging sequence correctly on the basis of information included in preambles. Particularly, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program that are robust against parity errors that occur in SIGNAL fields in preambles representing transmission rate, data length, etc.
2. Description of the Related Art
There exist increasing interests in wireless networks, which are free of wires used in wired communication systems. Examples of standards for wireless networks include the IEEE (the Institute of Electrical and Electronics Engineers) 802.11 or the IEEE 802.15.
For example, the IEEE 802.11a/g, which is a standard protocol for wireless LANs, employs orthogonal frequency division multiplexing (OFDM), which is a type of muiticarrier modulation. With OFDM, transmission data is carried on a plurality of carriers having mutually orthogonal frequencies. Thus, the bandwidth of each of the carriers is narrow, so that frequencies can be used efficiently, and robustness against frequency-selective fading is high.
The IEEE 802.11a/g supports a modulation scheme that allows a maximum communication rate of 54 Mbps. However, there exists a demand for a next-generation LAN standard that allows an even higher bitrate. For example, the IEEE 802.11n, which is an extended version of the IEEE 802.11a/g, aims at developing high-speed wireless LAN technologies that can achieve an effective throughput exceeding 100 Mbps by employing multi-input multi-output (MIMO) communications.
Furthermore, the PHY layer of the IEEE 802.11n defines a high-throughput (HT) transmission mode (hereinafter referred to as the “HT mode”) with a modulation and coding scheme (MCS) totally different from that of the IEEE 802.11a/g, as well as an operation mode in which data is transmitted with the same packet format and using the same frequency range as in the IEEE 802.11a/g (hereinafter referred to as the “legacy mode”). Furthermore, as a mode in the HT mode, an operation mode called the “mixed mode (MM)”, having compatibility with terminals compliant with the IEEE 802.11a/g (hereinafter referred to as “legacy terminals”), is defined. The mixed mode is described, for example, in the EWC (Enhanced Wireless Consortium) PHY Specification. The IEEE 802.11n requires support of the mixed mode (MM).
FIGS. 4 and 5 show packet formats in the legacy mode and the MM mode, respectively. In FIGS. 4 and 5, the duration of each OFDM symbol is 4 microseconds.
Referring to FIG. 4, the packet in the legacy mode (hereinafter referred to as a “legacy packet”) has the same format as a packet according to the IEEE 802.11a/g. A header of the legacy packet includes a legacy short training field (L-STF) containing known OFDM symbols for packet discovery, a legacy long training field (L-LTF) containing known training symbols for synchronization acquisition and equalization, and a legacy signal field (L-SIG) as a SIGNAL field representing a transmission rate, a data length, etc. The head is followed by a payload (DATA field).
On the other hand, the packet shown in FIG. 5 (hereinafter referred to as an “MM packet”) includes a legacy preamble having the same format as a preamble according to the IEEE 802.11a/g, a preamble defined by the IEEE 802.11n (hereinafter referred to as an “HT preamble”), and a data portion. The HT preamble and the data portion (shaded in FIG. 5) have an HT format, with which a communication scheme specific to the IEEE 802.11n is used.
The HT preamble includes HT-SIG, HT-STF, and HT-LTF. The HT-SIG is a SIGNAL field that is used in HT transmission in the MM mode. The HT-SIG includes information used to interpret the HT format, such as an MCS used for the PHY payload (PSDU) and the payload data length. The HT-STF contains training symbols for facilitating automatic gain control (AGC) in an MIMO system. The HT-LTF contains training symbols for executing channel estimation at a receiver for each input signal that is spatially modulated (mapped).
In the case of MIMO communications, in which two or more transmission branches are used, at a receiver, a channel matrix is obtained by estimating channel coefficients for each combination of transmission and reception antennas that spatially separate received signals. Thus, at a transmitter, HT-LTFs are transmitted from transmission antennas by time division, and one or more HT-LTF fields are added in accordance with the number of spatial streams.
The legacy preamble in the MM packet have the same format as the preamble in the legacy packet, and is transmitted in a format that can be decoded by legacy terminals. On the other hand, the HT format portion beginning with the HT preamble is transmitted in a transmission scheme not supported by legacy terminals. A legacy terminal decodes the L-SIG in the legacy preamble in the MM packet, finds that the packet is not addressed to the own terminal and reads data length information and so forth, and sets a network allocation vector (NAV) with an appropriate length, i.e., a transmission waiting period, thereby avoiding collision. As a result, the MM packet is compatible with legacy terminals.
A SIGNAL field in the preamble of a packet is a field describing information regarding the transmission rate and data length of the packet. In the legacy mode, an actual value is described in the L-SIG. On the other hand, in the MM mode, an actual value is described in the HT-SIG. FIG. 6 and FIGS. 7A and 7B show the formats of the L-SIG and HT-SIG fields.
In a packet transmitted in the legacy mode by an HT terminal compliant with the IEEE 802.11n (EWC), the L-SIG field is transmitted with the same definition and scheme as described in Section 17.3.4 of the IEEE 802.11a standard. On the other hand, the legacy preamble of the MM packet transmitted is the legacy mode is compatible with legacy terminals. Thus, it is not allowed to describe the actual (high-speed) transmission rate, which is not supported in the legacy mode. The L-SIG field is used to spoof the transmission rate (RATE) and the packet length (LENGTH) information to legacy terminals. More specifically, in the L-SIG, the RATE field contains a bit sequence representing 6 Mbps, which is compatible with legacy terminals, and the LENGTH field contains a value defining an appropriate network allocation vector (NAV) instead of an actual data length. That is, packet-length information is spoofed in the L-SIG determined in accordance with the transmission rate so that the value obtained by dividing the packet length by the transmission rate becomes equal to a desired duration for which communications are to be refrained.
Upon receiving an MM packet, a legacy terminal sets an NAV for a period of a duration that is determined by dividing the spoofed packet length by the transmission rate on the basis of the L-SIG field of the packet, and refrains from transmission during this period. Thus, collision with the MM packet is avoided, so that the packet exchanging sequence can be maintained.
Furthermore, among HT terminals, MM packets are mutually recognized in such a manner the HT terminals do not recognize that the transmission rate and packet length in the L-SIG field are spoofed to legacy terminals, so that the legacy terminals operate as specified in the L-SIG field. Thus, the HT-SIG is transmitted in such a scheme that the HT-SIG can be decoded by all HT terminals but will not be decoded by legacy terminals. More specifically, the HT-SIG field is BPSK-modulated (refer to FIGS. 8A and 8B) in a phase space that is rotated by 30 degrees relative to a phase space for the L-SIG field (or preceding or succeeding field). Upon detecting a packet, an HT terminal at a receiving end checks whether the absolute phase space of the fifth OFDM symbol is rotated by 90 degrees. When the phase of the symbol is rotated by 90 degrees, the HT terminal can determine that the field is an HT-SIG field and that the packet is an MM packet. This is described, for example, in Japanese Unexamined Patent Application Publication No. 2006-50526, paragraphs 0142 to 0147 and FIG. 15. Furthermore, similarly to a legacy terminal, the HT terminal calculates a period of NAV on the basis of the content of the L-SIG field and refrains from transmission during the period, so that the packet exchanging sequence can be maintained.
Referring to the format of the L-SIG field shown in FIG. 6, regarding error detection, only one bit at the 18th bit, which functions as a parity bit, is provided. In this case, errors that occur at an even number of bits are not detected (refer to FIG. 9), so that communications are susceptible to parity errors.
When errors that have occurred are not detected so that a NAV is set for a wrong period calculated on the basis of the incorrect RATE information and LENGTH information in the L-SIG field, the system throughput is reduced if transmission is refrained unnecessarily.
In the case of an HT packet, even when a parity error occurs in the L-SIG field, in some cases, it is possible to discard ail the received data in the field and to recover a normal communication sequence on the basis of the content of the succeeding HT-SIG field or MAC header. However, such a chance of recovery might be missed when errors that occur at an even number of bits are not detected.